Disk drive devices using various kinds of disks, such as optical disks, magneto-optical disks, flexible magnetic disks, and the like, have been known in the art. In particular, hard disk drives (HDDs) have been widely used as storage devices of computers and have been one of indispensable storage devices for current computer systems. Moreover, the HDDs have found widespread application to moving image recording/reproducing apparatuses, car navigation systems, cellular phones, and the like, in addition to the computers, due to their outstanding characteristics.
A magnetic disk used in a HDD has multiple concentric data tracks and servo tracks. Each servo track is constituted by a plurality of servo data containing address information. Each data track includes multiple data sectors containing user data recorded thereon. Data sectors are recorded between servo data located discretely in the circumferential direction. A head element portion of a head slider supported by a swing actuator accesses a desired data sector in accordance with address information in the servo data to write data to and retrieve data from the data sector.
In order to increase recording density of a magnetic disk, it is important to decrease the clearance between the head element portion flying over the magnetic disk and the magnetic disk and to decrease the variation of the clearance; some mechanisms have been proposed to control the clearance. One of such mechanisms has a heater which heats the head element portion to adjust the clearance in a head slider (for example, refer to Japanese Patent Publication No. 2007-80409 “Patent Document 1”). In the present specification, it is called thermal fly-height control (TFC). The TFC generates heat by applying electric current to the heater to make the head element portion protrude by thermal expansion. This reduces the clearance between the magnetic disk and the head element portion.
The TFC adjusts the protruding amount of a head element portion by controlling the heat generation amount of a heater. A clearance is a spacing between the head element portion and a magnetic disk; the relationship between the heater power and the clearance depends on the shape of the head element portion at a heater OFF state. Specifically, a clearance varies depending on the initial position of the end surface of the head element portion facing the magnetic disk even if the protruding amounts are the same. For precise clearance control, it is demanded to supply heater power depending on the initial shape (the shape at the heater OFF) of the head element portion.
Typically, a head element portion on a head slider is recessed from the air bearing surface (ABS) of a slider. The distance between the ABS and the initial position of the end surface of the head element portion facing the magnetic disk is called a recess depth. The recess depth is defined by grinding and etching steps in manufacturing a head slider. Accordingly, variations in recess depth among head sliders cannot be eliminated. Besides, the head slider is bonded to an actuator and mounted in an HDD. There exists fabrication tolerance in fabricating an HDD, and variations in the fly-height is different in each head slider.
Since the TFC can adjust the clearance (protruding amount of the head element portion) for each head slider, it can reduce variations in recess depth and fly-height of slider to perform appropriate clearance control for each head slider. However, if the recess depth is too large or the initial clearance (the sum of the fly-height of a slider and the recess depth) of a head slider mounted on an HDD is too large, the heater power to be applied for attaining a desired clearance becomes too much to perform accurate clearance control.
In a typical HDD, the maximum heater power value in TFC is specified in order to prevent damage onto the head element portion. That is, the protruding amount of the head element portion using a heater is limited. Therefore, if the recess depth or the initial clearance of head slider is large, sufficiently small clearance cannot be attained so that an accurate read/write operation may not be able to be performed. Consequently, a technique has been demanded that can adjust a recess depth to a smaller depth if the recess depth of a head slider is too large.